URL Envy by Valera Fooksman

URL Envy allows editing URL parameters in the left sidebar, one key/value pair per row. You can edit the keys/values, remove key/value pairs, add new ones, or change the base URL. You can also sort the parameters by key.

Both GET and POST methods are supported since v.1.1.1.

URL Envy is based on FireFox's Web Extensions API which is the new standard starting from FireFox v.60. The older style extensions no longer work after this FireFox version. That's why URL Shooter (https://github.com/vmeurisse/url-shooter) by Vincent Meurisse, the extension I used previously for the same purpose, had to be replaced. URL Shooter, in turn, was a replacement for an even older URLParams by Erik Schmidt.

The extension is invoked by pressing Ctrl+Shift+U (on Mac, Command+Shift+U).

About permissions
Version 1.1.1 asks for two permissions the earlier version did not. Firefox describes them in broad terms, so here is what they are actually for.

• Access your data for all websites. URL Envy edits the address of whatever page you are on, so it has to work on any site. It also needs this to send a request as POST: the parameters cannot travel in the address, so URL Envy builds a form in the page you are already on and submits it from there, which keeps the request on the same site. It does not read the content of pages.
• Access recently closed tabs. This is Firefox's name for the "sessions" permission, which lets an extension attach its own notes to a tab. URL Envy uses it to remember the parameters and the GET/POST choice per tab, so they are still there after a browser restart. It does not look at your browsing history or your closed tabs.

URL Envy collects no data. Nothing is sent to the author or to any third party — only the request you asked it to send, to the site you were already querying.